The present invention relates to the operation of subsystems of a disc drive. More particularly, the present invention relates to a programmable integrated filter for disc drive subsystems.
A magnetic disc, commonly used in a computer disc drive, is a flat circular platter with a magnetic surface on which data can be stored by selective polarization of portions of the magnetic surface. The presence or absence of polarity transitions between the polarized portions represents particular binary values. Typically, the magnetically polarized portions are arranged in a plurality of radially concentric tracks on the surface of the disc to aid in location and read back of the data.
As a magnetic transducer moves relative to the magnetic disc, it generates an electrical pulse each time it encounters a polarity transition on the disc. These electrical pulses create a read signal. The data within the read signal is read back, or recovered, by sending the signal through various subsystems of the disc drive. These subsystems typically include amplifiers, filters, and logic circuits.
For example, the incoming read signal often has varying peak-to-peak amplitude due to temperature variation, variations in the media surface, different characteristics of different magnetic transducers, and variations in the flying height of the magnetic transducers. In order to keep the amplitude within a desirable range, the read signal is provided to a feedback loop referred to as an automatic gain control (AGC) subsystem. The read signal is first fed to a variable-gain amplifier (VGA) whose gain is controlled by an electrical input. The read signal amplitude is then typically sampled by an "envelope detector", compared to a preassigned set point, and the difference (which represents an error signal) is fed back to the VGA. In this way, the output signal from the AGC subsystem can be regulated to maintain the amplitude of the read signal at a fixed level or within a desired range.
In accomplishing automatic gain control, the AGC subsystem typically includes (in addition to the VGA) a full waive rectifier and a low-pass, first order, R-C passive filter with a large discrete capacitor. Typically, the capacitor for the low-pass filter is so large that it must be a discrete component. The large size of the capacitor creates at least two problems. The first is that the discrete capacitor prohibits fully integrating the AGC subsystem. The second is that the fixed nature of the discrete capacitor restricts the efficient operation of the AGC subsystem to a single speed of operation.
After the read signal exits the AGC subsystem it is sent to a pulse detector subsystem in order to synchronize logic level transitions with the peaks of the read signal. Ideally, each electrical pulse in the read signal corresponds to a data bit on the disc. However, additional electrical pulses can be erroneously created between the data bits. These additional pulses can be caused by a number of events including: noise in the read back circuit, noise in the write circuit when data is written to the disc and spurious polarity transitions on the disc. In order for the data in the read signal to be useful, these additional, erroneous pulses must be eliminated from the read signal.
One way to eliminate additional pulses is to generate a series of read windows which represent time frames in which data pulses are expected. Pulses in the read signal which occur outside of these read windows are then filtered out of the read signal. In current disc drive systems, this technique is accomplished using a phase locked loop subsystem and a data separator. The phase locked loop generates the read windows and the data separator filters out the additional pulses. The phase locked loop generates the read windows by producing a clock signal with a frequency based on the frequency of pulses contained in the read signal.
The phase locked loop is capable of automatic adjustment, and it continually attempts to acquire a new frequency if the frequency of the pulses in the read signal changes. How quickly the phase locked loop adjusts to new frequencies affects the data quality and the data retrieval speed of the disc drive.
The data quality is determined by how much noise is present in the read signal as it leaves the data separator. While the disc drive is reading the data, it is said to be in the read mode. If the phase locked loop, while in the read mode, adjusts too quickly to changes in frequency, the data separator can interpret noise as data pulses. For instance, if there are four data pulses per second, a series of eight data pulses with a noise pulse between the seventh and eighth pulse will be interpreted as six pulses at a frequency of four pulses per second followed by three pulses at a frequency of six pulses per second. Therefore, when the disc drive is in the read mode, it is advantageous to have the phase locked loop not react too quickly to changes in frequency.
The data retrieval speed is affected by how quickly the phase locked loop can lock on to a clock frequency for a read signal from a different track when the transducer switches between tracks which have data recorded at different frequencies. While the disc drive is attempting to recover the clock signal, it is said to be in fast acquisition mode. If the phase locked loop, while in fast acquisition mode, adjusts too slowly to changes in frequency, the disc drive retrieval time increases. Therefore, it is advantageous to have the phase locked loop react quickly to changes in frequency when the disc drive is operating in fast acquisition mode.
Within the phase locked loop subsystem, the loop filter determines how quickly the phase locked loop adjusts itself based on changes of frequency in the read signal. The reaction time of the phase locked loop is controlled by a capacitor within the loop filter. By changing the capacitor, the loop filter reaction time can be changed. Increasing the size of the capacitor slows the reaction time of the loop filter while decreasing the size of the capacitor speeds up the reaction time of the loop filter.
Typically, the capacitor for the loop filter is so large that it must be a discrete component. The large size of the capacitor causes the same two problems as the discrete capacitor in the AGC subsystem. That is, the discrete capacitor prohibits fully integrating the phase locked loop, and the fixed nature of the discrete capacitor restricts the loop filter to operating efficiently in only one mode, either the read mode or the fast acquisition mode.
The inability to fully integrate the AGC subsystem and the phase locked loop has many negative effects. Discrete components take more board space and thus cause the disc drive to be larger. In addition, since the rest of the AGC subsystem and the phase locked loop is integrated, the discrete components require a second manufacturing step. This second manufacturing step increases drive costs and reduces quality.
The fixed nature of the capacitors also have negative effects. In order to achieve maximum efficiency in the phase locked loop and AGC subsystem, two filters must be built. Depending on the mode of operation, the appropriate filter must be switched into the phase locked loop or AGC subsystem. The two filters consume a large amount of space. The increased number of components also increases the likelihood of defects. It is also cumbersome to provide the switching circuitry necessary to switch between the two filters. Thus, both cost and quality are negatively affected by the fixed nature of the capacitor.
Other subsystems of the magnetic disc drive, for example, the servo subsystem, utilize filters with discrete components. In particular, they often use large capacitors in order to provide proper filtering actions for the subsystem. The use of these components in each of these subsystems presents the same problems as in the AGC subsystem and the phase lock loop subsystem. Discrete components take up more board space and thus cause the disc drive to be larger. In addition, since most of the subsystems are integrated, the discrete components require a second manufacturing step. This second manufacturing step increases drive costs and reduces quality.